This invention relates generally to helium compressor units for use in cryogenic refrigeration systems, operating on the Gifford McMahon (GM) cycle. More particularly, the invention relates to an improved oil cooling structure for a scroll type oil-lubricated compressor unit adapted to compressing helium by orienting it horizontally.
A refrigeration compressor has a need for lubrication of moving parts such as bearings and gears. These compressors contain oil sumps to direct oil from the sump to each lubrication point. Oil-lubricated air conditioning compressors have become standard for delivering pressurized helium to GM type cryogenic refrigerators. The ability to use these relatively inexpensive but reliable compressors results from developing methods to cool the helium as it is being compressed, and the development of oil separators and adsorbers that reliably keep oil out of the cold expander of a GM type refrigeration system. Because helium gets much hotter during compression than standard air-conditioning refrigerants it is frequently cooled by flowing a significant amount of oil along with the helium through the compression chamber. Additionally, the compressor units also generate heat in the compression of helium. Therefore, the purpose of oil in GM type cryogenic refrigeration is both lubrication and to absorb the heat produced in the process of helium compression.
The basic principal of operation of a GM cycle refrigerator is described in U.S. Pat. No. 2,906,101 to McMahon, et al. The GM cycle has become the dominant means of producing cryogenic temperatures in small commercial refrigerators primarily because it can utilize mass produced oil-lubricated air-conditioning compressors to build reliable, long life, refrigerators at minimal cost. GM cycle refrigerators operate well at pressures and power inputs within the design limits of air-conditioning compressors, even though helium is substituted for the design refrigerants. Typically, GM refrigerators operate at a high pressure (Ph) of about 2 MPa (300 pounds per square inch absolute) (psia), and a low pressure of about 0.8 MPa (117 psia).
Air-conditioning compressors are built in a wide range of sizes and several different designs. Means of providing additional cooling to adapt these compressors to compressing helium are different for different compressors. For example, compressors that draw approximately 200 to 600 W are typically reciprocating piston types which are cooled by adding air cooled fins to the compressor shell. Between about 800 to 4,500 W, the most common compressor is a rolling piston type with low pressure return gas flowing directly onto the compression chamber. In rolling piston compressors, oil flows into the compression chamber along with the helium and absorbs heat from the helium as it is being compressed. Most of the oil separates from the helium in the compressor shell which is at high pressure. U.S. Pat. No. 6,488,120 to Longsworth describes the cooling of helium, oil, and the compressor shell by wrapping a water cooling tube around the shell, and further wrapping a helium cooling tube and an oil cooling tube over the water tube. Cooled oil is then injected into the return helium line. In effect, the compressor serves as an oil pump. The amount of oil pumped is typically about 2% of the displacement.
A problem with the oil cooling system is the flow rate and temperature of the cooling water are very important and must be monitored carefully. Failure to monitor reduces the effectiveness of the oil separators, causes overheating, and increases the likelihood of compressor shutdown or failure.
The Hitachi Corporation manufactures scroll compressors which draw between 5 and 9 kW and have return gas flow directly into the scroll. Oil can be injected into the inlet and discharged with the helium into the shell at high pressure. Most of the oil separates from the helium and collects in the bottom of the compressor, similar to the rolling piston compressor described above. Unlike the smaller compressor, for this type of compressor, cooling the shell with a water cooling tube wrapped around it is not effective. Here, heat from the helium and oil is removed by an after-cooler that is either air or water cooled.
The Copeland Compressor Corporation manufactures scroll compressors for air-conditioning service that draw between 5 and 15 kW. These compressors differ from the Hitachi design in that the return gas flows into the shell, which is at low pressure, rather than directly into the scroll. In the standard vertical orientation, in which the scroll is above the motor, no means exist to have cooling oil flow into the compression chamber with the helium. Copeland has modified two compressors, a 5 and a 7.5 kW compressor, to circulate oil for cooling helium by collecting high pressure oil in the discharge plenum above the scroll then having it flow out through a special port to be cooled in an external after-cooler. Another special return port brings oil back into the scroll near low pressure where it mixes with helium that is being compressed.
A description of the construction and operation of a scroll compressor, and the specific changes to adapt the Copeland standard unit to compressing helium, is found in U.S. Pat. No. 6,017,205 to Weatherston, et al. A compressor system that uses the larger of the two compressors that are manufactured for helium service together with a description of the entire compressor system, of which the compressor is an essential component, is described in R. C. Longsworth, “Helium Compressor for GM and Pulse-tube Expanders”, in “Advances in Cryogenic Engineering”, Vol. 47, Amer. Inst. of Physics, 2002, pp 691-697.
In an effort to reduce the cost of applying the above scroll compressors to applications that require oil injection for cooling, Copeland successfully oriented the compressors horizontally. In the Copeland compressor, oil in the bottom of the compressor at low pressure flows into the scroll due to gravity along with the gas being compressed. The only modification to a standard vertical compressor is the addition of a port at the bottom center of the compressor. In the horizontal orientation, oil, which would normally be pumped from the oil sump in the bottom of the compressor up the drive shaft to lubricate the bearings and scroll, is directed at the end of the drive shaft after it is cooled in an after-cooler. More oil flows into the scroll with the helium than when oriented vertically. However, a problem with the horizontal orientation is that more oil is circulated than is needed to lubricate the bearings and the “excess” collects in the bottom of the shell. The excess oil flows through the “air” gap in the motor to the scroll, thereby putting significant drag on the motor.
When a standard Copeland scroll compressor is operated horizontally, the cooling oil directed into the end of the drive shaft contains a large fraction of oil in excess of the amount needed to lubricate the bearings. The excess falls to the bottom of the compressor shell and much of it flows through the “air” gap between the rotor and stator to get to the scroll inlet, where it is pumped along with the helium to high pressure. The oil in the “air” gap and the resultant drag causes the motor to draw more power than when the compressor is operated in the vertical position.
A further problem with the horizontal orientation is greater vibration. In addition to inherent vibration from the compressor, operating the standard Copeland scroll compressor horizontally, results in even greater vibration due to oil in the “air” gap.
Accordingly, there exists a need to improve the oil cooling system of Copeland type horizontally oriented oil-lubricated compressors. The present invention is made in view of the above described problems. It is, therefore, desirable to have a oil-lubricated compressor that reduces drag on the motor. It is also desirable to have an efficient oil-lubricated compressor utilizing reduced input power, that can be operated at variable speeds, and having reduced vibration.
None of the references disclose an oil bypass such that when the oil returning from the after-cooler is divided into two streams, one that lubricates the bearings plus an excess that drops to a sump, and a second that bypasses the motor in a tube that is external to the compressor shell and flows back into the shell near the scroll inlet, the input power is reduced by a significant amount.